The present invention relates to fuel injection control device and method for an internal combustion engine applied to an internal combustion engine that includes two types of fuel injection valves, which include a direct injection valve, which injects fuel into a cylinder, and port injection valve, which injects fuel into an intake port.
The internal combustion engine that includes the above-described two types of fuel injection valves allows the injection mode to be selected among a port-injection-only mode, in which only the port injection valve performs fuel injection, a direct-injection-only mode, in which only the direct injection valve performs fuel injection, and a distributed injection mode, in which both the fuel injection valve performs fuel injection. In the fuel injection control device disclosed in Japanese Laid-Open Patent Publication No. 2013-209935, such selection of the injection mode is performed based on the coolant temperature. More specifically, if the coolant temperature is lower than or equal to a cold temperature, the port-injection-only mode is selected as the injection mode. If the coolant temperature is in the range from the cold temperature to a warm-up completion temperature, the direct-injection-only mode is selected as the injection mode. If the coolant temperature is higher than or equal to the warm-up completion temperature, the distributed injection mode is selected as the injection mode.
The above-described cold temperature is set to the lower limit value of the coolant temperature at which poor vaporization of fuel can be kept within a permissible range. The poor vaporization of fuel is caused by adhesion of fuel to the piston and the wall surface of the cylinder when fuel is injected through the direct injection valve. That is, the above-described conventional fuel injection control device switches the injection mode from the port-injection-only mode to the direct-injection-only mode if the wall temperature of the piston and the cylinder grasped from the coolant temperature is increased to a level sufficient to keep the poor vaporization, which is caused by the adhesion of fuel, within the permissible range.
If the outside air is at an extremely low temperature, the temperature of intake air flowing through the intake port is also decreased, and the intake air cools the wall surface of the intake port. Thus, although the coolant temperature is increased, the wall temperature of the intake port may sometimes be kept low. If fuel injection by the port injection valve (port injection) is performed in such a case, adhesion of fuel to the wall surface of the intake port is increased, and the amount of fuel burned in the combustion chamber is decreased accordingly. This may possibly degrade the combustion.